U.S. patent number 5,134,997 [Application Number 07/567,204] was granted by the patent office on 1992-08-04 for rate responsive pacemaker and pacing method.
This patent grant is currently assigned to Medtronic, Inc.. Invention is credited to Tommy D. Bennett, Lucy M. Nichols, Glenn M. Roline, David L. Thompson.
United States Patent |
5,134,997 |
Bennett , et al. |
August 4, 1992 |
Rate responsive pacemaker and pacing method
Abstract
A rate responsive pacemaker and a pacing method for optimizing
the pacing decay curve after a period of increased activity. The
pacing method includes the steps of selecting a set of
predetermined achievement criteria such as an achievement rate and
an achievement time interval. The achievement rate is selected
between an upper pacing rate and a first pacing switch rate
threshold. The pacing method then determines whether the
achievement criterion has been met. If the achievement criterion
has been met, then the decay time constant of the decay curve
changes from a first value to a second value, as the pacing rate
drops below the first pacing switch rate threshold. A second pacing
switch rate threshold lower than the first pacing switch rate
threshold is then selected, and, if the achievement criteria have
been met, then the decay time constant of the decay curve is
modified from the second value to a third value, as the pacing rate
drops below the second pacing switch rate threshold.
Inventors: |
Bennett; Tommy D. (Shoreview,
MN), Nichols; Lucy M. (Maple Grove, MN), Roline; Glenn
M. (Anoka, MN), Thompson; David L. (Fridley, MN) |
Assignee: |
Medtronic, Inc. (Minneapolis,
MN)
|
Family
ID: |
24266170 |
Appl.
No.: |
07/567,204 |
Filed: |
August 14, 1990 |
Current U.S.
Class: |
607/19;
607/18 |
Current CPC
Class: |
A61N
1/36585 (20130101); A61N 1/37 (20130101) |
Current International
Class: |
A61N
1/365 (20060101); A61N 1/362 (20060101); A61N
1/37 (20060101); A61N 001/362 () |
Field of
Search: |
;128/419PG |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kamm; William E.
Assistant Examiner: Schaetzle; Kennedy J.
Attorney, Agent or Firm: Rissman; John A. Patton; Harold
R.
Claims
What is claimed is:
1. Method for pacing along a decay curve having a decay time
constant, the pacing method comprising the steps of:
a. selecting a set of predetermined achievement criteria;
b. selecting a first pacing switch rate threshold;
c. determining whether said achievement criteria have been met;
and
d. if said achievement criteria have been met, then modifying the
decay time constant of the decay curve from a first value to a
second value, as the pacing rate drops below said first pacing
switch rate threshold, such that said second value is different
from said first value.
2. The method as defined in claim 1, further including the steps
of:
a. selecting a second pacing switch rate threshold lower than said
first pacing switch rate threshold; and
if said achievement criteria have been met, then modifying the
decay time constant of the decay curve from the second value to a
third value, as the pacing rate drops below said second pacing
switch rate threshold, such that said second value is different
from said third value.
3. The method as defined in claim 2, wherein said second value of
the time constant for the decay curve is longer than said first
value, in order to allow a slower decay of the pacing rate.
4. The method as defined in claim 3, wherein said third value of
the 5 time constant for the decay curve is substantially equal to
said first value.
5. The method as defined in claim 1, further including the step of
keeping the time constant of the decay curve unchanged if said
achievement criteria have not been met.
6. The method as defined in claim 1, wherein said achievement
criteria includes an achievement rate and an achievement time
interval; wherein said achievement rate is a programmable
percentage of the difference between a Lower Rate (LR) and an Upper
Rate (UR); wherein said Lower Rate and Upper Rate are the lower and
upper pacing rates respectively; and wherein the achievement time
interval is a minimum time interval over which the pacing rate
exceeds said achievement rate.
7. The method as defined in claim 6 wherein said achievement time
interval is about 4 seconds.
8. The method as defined in claim 6, wherein said achievement rate
is calculated as follows:
where "a" is a percentile value which ranges between 50% and 100%,
and the Lower Rate and the Upper Rate are the lower and upper
pacing rates respectively.
9. The method as defined in claim 6, wherein said achievement rate
is 125 pulses per minute.
10. The method as defined in claim 2, wherein said first pacing
switch rate threshold is calculated as follows:
where "u" is a percentile value which ranges between 20% and 50%,
and where the Lower Rate and the Upper Rate are the lower and upper
pacing rates respectively.
11. The method as defined in claim 10, wherein said first pacing
switch rate is about 90 pulses per minute.
12. The method as defined in claim 10, wherein said second pacing
switch rate threshold is calculated as follows:
13. The method as defined in claim 12, wherein said second pacing
switch rate is about 77 pulses per minute.
14. The method as defined in claim 1, further including the steps
of:
a. using an activity sensor to sense cardiac events, and to
generate an output indicative of the amplitude of the sensed
cardiac events; and
b. calculating a target rate, wherein said step of calculating said
target rate, is performed according to the following equation:
##EQU3## where C and D are selectively programmable values, and
where said Activity Count is a measure of said activity sensor
output over a predetermined interval of time.
15. A cardiac pacemaker of the type having a pulse generator for
generating stimulus pulses, comprising: means for pacing along a
decay curve having a decay time constant, comprising:
i) means for selecting a set of predetermined achievement
criterion;
ii) means for selecting a first pacing switch rate threshold;
iii) means for determining whether said achievement criterion has
been met; and
iv) means for modifying the decay time constant of the decay curve
from a first value to a second value, as the pacing rate drops
below said first pacing switch rate threshold, such that said
second value is different from said first value, if said
achievement criterion has been met.
16. The cardiac pacemaker as defined in claim 15, wherein said
pacing means further includes:
a. means for selecting a second pacing switch rate threshold lower
than said first pacing switch rate threshold; and
b. means for modifying the decay time constant of the decay curve
from the second value to a third value, if said achievement
criteria have been met, as the pacing rate drops below said second
pacing switch rate threshold, such that said second value is
different from said third value.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to medical cardiac pacers,
and more particularly, it relates to a pacer of the type which
responds to the patient's metabolic demand and varies the decay
rates in substantial similarity to the heart's normal behavior.
2. Description of the Prior Art
Early cardiac pacemakers provided a fixed-rate stimulation pulse
generator that could be reset, on demand, by sensed atrial and/or
ventricular depolarizations. Modern pacemakers include complex
simulation pulse generators, sense amplifiers and leads which can
be configured or programmed to operate in single or dual chamber
modes of operation, delivering pacing stimuli to the atrium and/or
ventricle at fixed rates or rates that vary between an upper rate
limit and a lower rate limit.
In recent years, single and dual chamber pacemakers have been
developed which measure parameters which are directly or indirectly
related to metabolic requirements (e.g., demand oxygenated blood)
and vary the pacing rate in response to such parameters. Such
measured parameters include, for example, physical activity of the
body, right ventricular blood pressure and the change of right
ventricular blood pressure over time, venous blood temperature,
venous blood oxygen satuation, respiration, minute ventilation, and
various pre and post-systolic time intervals measured by impedance
or pressure sensing within the right ventricle of the heart. Such
sensor-driven pacemakers have been developed for the purpose of
restoring rate response to exercise in patients lacking the ability
to increase rate adequately by exertion.
In general, a rate responsive pacemaker includes a sensor which
produces an output that varies between a maximum sensor output
level and a minimum sensor output level ("Sensor Output"), and a
packing rate is provided by the pacemaker ("Pacing Rate") which
typically varies as a linear or monotonic function ("f") of the
sensor output, between a selectable lower pacing rate ("Lower
Rate") and upper pacing rate ("Upper Rate"). Function f has a
selectable slope (i.e., Pacing Rate change/Sensor Output Change)
adjustable by means of an external programmer in conjunction with
the Lower and Upper Rates. Thus, the Pacing Rate typically provided
is equal to the pre-selected Lower Rate plus an increment which is
a function of the measured Sensor Output, as follows:
While this rate response technique provides a useful and workable
system between the programmed parameters, the behavior of the
pacemaker is complex and not often readily apprehended. Among these
rate responsive pacemakers, those that measure the physical
activity of the patient by means of a piezoelectric transducer have
become popular among the various rate responsive pacemakers. Such
an activity rate responsive pacemaker is described in U.S. Pat. No.
4,485,813 issued to Anderson et al.
Some temperature sensing pacemakers have employed relatively more
complex functions to take into account the initial dip in
temperature due to the onset of exercise. One such pacemaker is
described in U.S. Pat. No. 4,719,920 issued to Alt.
Furthermore, the decay slope of conventional pacemakers do not
approximate the heart's normal behavior, in that they are
programmed to follow a curve based on a single time constant. This
discrepancy between the normal heart deceleration function at the
end of physiologic stresses, such as physical activity, and the
conventional decay function has not been totally rectified by any
pacemaker presently available on the market.
Wherefore, it is desirable to have a new cardiac pacemaker and
method of pacing with activity or other rate responsive dependent
parameters, for responding to the patient's metabolic demand and
for varying the attack and decay pacing rates in harmony with the
heart's normal behavior.
BRIEF SUMMARY OF THE INVENTION
It is therefore one object of the present invention to address the
above problems and to provide adequate solutions thereto.
Briefly, the above and further objects and features of the present
invention are realized by providing a rate responsive pacemakers
and a pacing method for optimizing the pacing decay curve after a
period of increased activity. The pacing method includes the steps
of selecting a set of predetermined achievement criteria such as an
achievement rate and an achievement duration or time interval.
The achievement rate is initially selected between an upper pacing
rate and a first pacing switch rate threshold. The pacing method
then determines whether the achievement criterion has been met. If
the achievement criterion has been met, then the decay time
constant of the decay curve changes from a first value to a second
value, as the pacing rate drops below the first pacing switch rate
threshold.
A second pacing switch rate threshold lower than the first pacing
switch rate threshold is then selected, and, if the achievement
criterion has been met, then the decay time constant of the decay
curve is modified from the second value to a third value, as the
pacing rate drops below the second pacing switch rate threshold.
The second value of the time constant for the decay curve should be
longer than the value, in order to allow a slower decay of the
pacing rate. In the preferred embodiment of the present invention,
the third value of the time constant for the decay curve is made
substantially equal to the first value.
If on the other hand the achievement criteria have not been met,
then the time constant of the decay curve is not modified.
The cardiac pacemaker also periodically calculates the new activity
pacing rate, and then calculates the new activity target rate based
upon the activity sensor output. In the preferred embodiment, the
achievement rate is calculated as follows:
10 where "a" is a percentile value.
The first or upper pacing switch rate threshold is calculated as
follows:
where "u" is a percentile value.
The second or lower pacing switch rate threshold is calculated as
follows:
The target rate is calculated according to the following equation:
##EQU1## In the above equations a, u, C, D, Lower Rate and Upper
Rate are selectively programmable values.
By using the above inventive pacing method, the pacemaker responds
to the patient's metabolic demand and varies the decay rates in
harmony with the heart's normal behavior, and allows for a gradual
decrement in the pacing rate during the decay or deceleration
period.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and features of the present invention
and the manner of attaining them, will become apparent, and the
invention itself will be best understood, by reference to the
following description and the accompanying drawings, wherein:
FIG. 1 graphically illustrates a multi-sensor pacemaker employing
the present invention;
FIG. 2 is a graph illustrating the behavior of the pacemaker
employing the present invention in comparison to the behavior of a
conventional pacemaker;
FIG. 3 is another graph further illustrating the behavior of the
pacemaker employing the present invention;
FIG. 4 is a flow chart illustrating the operation of the pacemaker
according to the present invention;
FIG. 5 is a block diagram illustration of a simplified cricuit for
the pacemaker according to the present invention; and
FIG. 6 is a graph of various rate response curves used in the
pacemaker of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings and more particularly to FIG. 1
thereof, there is illustrated a multi-sensor pacemaker 100
according to the present invention. The preferred embodiment of the
pacemaker 100 includes two sensors, and activity sensor 102 secured
to a can or housing 104, and a pressure sensor 106 implanted within
the patient's heart 108.
The operation of the sensors 102 and 106 is described in greater
detail in two co-pending U.S. patent applications Ser. No.
07/567,476, entitled "OPTIMIZATION FOR RATE RESPONSE CARDIAC
PACEMAKER", and Ser. No. 07/567,882, entitled "RATE RESPONSIVE
PACEMAKER AND METHODS FOR OPTIMIZING ITS OPERATION", both filed on
even date herewith and are incorporated herein by reference. It
should be understood that the present invention is not limited to a
dual sensor pacemaker, and that other sensors beside activity and
pressure sensors could also be used according to the present
invention. Nor is the present invention limited to a single chamber
pacemaker. A multi-chamber (i.e. dual chamber pacemaker) can also
be used in conjunction with the present invention. It should also
be understood that while the present invention will be described in
relation to the decay curve within the context of an activity-based
rate responsive pacemaker, the inventive concept can be
extrapolated for the attack curve, as well as for use in pressure
or multi-sensor pacemaker.
The distinction between the behavior of a pacemaker employing the
present invention and one according to the prior art is best
understood by consideration of FIG. 2. FIG. 2 illustrates a pacing
rate curve in a conventional activity sensing rate responsive
pacemaker as compared to a normal heart decay curve. The vertical
axis represents the pacing rate in pulses per minute (ppm), and the
horizontal axis represents time in seconds.
In this FIG. 2, the patient is initially at rest, as indicated by
the line 212, and the pacemaker is pacing at a predetermined lower
rate. The deflection point 214 indicates that the patient has
started to increase his or her activity level, and the attach or
acceleration curve 216 shows the pacemaker responding to such
increased activity level.
When the attach curve reaches a plateau 218, the pacing rate
generally stabilizes at an activity determined rate or an upper
rate for the duration of the exercise or physical activity. A
deflection point 220 indicates that the patient's activity level
has ceased or has been reduced substantially, and that the
pacemaker is now ready to decelerate the pacing rate.
Two decay or deceleration curves 222 and 224 descend from the
deflection point 220 and indicate a decrease in the patient's
activity level. In the absence of intervening heightened
activities, these two curves 222 and 224 tend to approach a
predetermined pacing rate, such as the lower rate.
The decay curve 222 represents the deceleration curve in a
conventional pacemaker, as exemplified in U.S. Pat. No. 4,722,342
issued to Amundson. The decay curve 224 on the other hand
represents the heart's normal deceleration rate, as illustrated in
a textbook by Myrvin H. Ellestad, M. D., entitled "Stress Testing
Principles and Practice", pages 489-492.
It is apparent that the curves 222 and 224 do not match completely
in that conventional pacemakers pace at an elevated rate, i.e.
curve 222, with respect to the typical human response, i.e. curve
228, and thereafter return to the resting or lower rate sooner than
the typical human response 230. This elevated pacing rate in
conventional pacemakers may cause a sensation of the heart rate
"racing" or beating too fast at the end of activity, perhaps even
provoking a syncopal episode. Additionally, conventional pacemakers
may pace too slowly for several minutes after the end of
activity.
The curve 224 comprises two decay portions, an initial portion 228
and a latent portion 230, each decaying at a different time
constant. As will be further explained in more details with respect
to FIGS. 3 and 4, the selection of the switch point 226 and the
time constants of the initial and latent decay portions 228 and 230
is an important part of the invention.
The following definition of terms used herein will assist in a
better understanding of the present invention:
Achievement Criterion--A value supplied by the clinician which sets
an attainment threshold for the Pacing Rate. This threshold
comprises a rate component (Achievement Rate) and a time component
(Achievement Duraton). The Achievement Rate is a programmable
percentage of the difference between the Lower Rate (LR) and the
Upper Rate (UR). The Achievement Duration is a minimum time
interval over which the Sensor Pacing Rate must exceed the
Achievement Rate. With rate response, the allowed programmable
values for the Achievement Criterion range from 70 ppm to 175 ppm
at 1 ppm intervals, and the Achievement Duration in this embodiment
is fixed at a four-second interval.
Activity Count--A measure of the output of the activity sensor over
a predetermined interval of time. In the preferred embodiment, each
event in which the amplitude of the output exceeds a predetermined
Activity Threshold for a two-second period is counted and retained.
The Activity Count is updated every two-second cycle, and its
aggregate value comprising the count value accumulated at the end
of 3 two-second cycles (i.e., after 6 seconds) is used to calculate
the sensor Target Rate for activity.
Activity Rate Response Gain--A setting which corresponds to the
slope of the function correlating the activity-based sensor Target
Rate of the Activity Count value which corresponds to the activity
sensor output. The setting for Activity Rate Response Gain,
sometimes alternately referred to as the "activity sensor gain",
corresponds to a particular rate response curve (RR). With rate
response, the allowed programmable values for the Activity Rate
Response Gain range from 1 and 10 at setting intervals of 1.
Activity Response Time Acceleration Constant--A value which
restricts the rate at which the activity-based sensor Pacing Rate
can increase, such that an activity "attack" curve provides for a
more gradual and physiologically appropriate change in pacing rate.
In the preferred embodiment, these time values represent the time
required to reach 90% of the difference between a first
steady-state activity-driven pacing period (constant activity
signal input for at least a six-second interval) and a second,
shorter, steady-state, activity-driven pacing period when a step
increase in activity level occurs. With rate response, the allowed
programmable values for the Activity Response Time Acceleration
Constant are selected from those of 0.25 minutes, 0.5 minutes, or
1.2 minutes.
Activity Response Time Deceleration Constant--A value which
restricts the rate at which the activity-based sensor Pacing Rate
can decrease, such that an activity "decay" curve provides for a
more gradual and physiologically appropriate change in pacing rate.
In the preferred embodiment, these time values represent the time
required to reach 90% of the difference between a first
steady-state activity-driven pacing period (constant activity
signal input for at least a six-second interval) and a second,
longer, steady-state, activity-driven pacing period when a step
decrease in activity level occurs. With rate response, the allowed
programmable values for the Activity Response Time Deceleration
Constant are selected from those of 2.5 minutes, 5 minutes, or 10
minutes.
Activity Threshold--A minimum value which the amplitude of the
activity sensor output must exceed to serve as input to the rate
determination algorithm. The higher the threshold, the greater the
amplitude necessary to become an event counted in the Activity
Count. With rate response, the allowed programmable values for the
Activity Threshold range from LOW, MEDIUM LOW, MEDIUM, MEDIUM HIGH,
and HIGH.
Lower Rate (LR)--A value supplied by the clinician which
establishes a lower boundary on the pacing rate. If the sensor is
disabled, or its sensor output is not large enough to increase
rate, the lower rate is the stimulus rate. With rate response, the
allowed programmable values for LR range from 40 ppm to 100 ppm at
1 ppm intervals.
Pacing Rate--The rate calculated by the pacemaker 100 in
conjunction with the activity sensor based upon its respective
Target Rate and the contribution thereto based upon its respective
acceleration and deceleration function.
Target Rate--The rate calculated by the pacemaker 100 in
conjunction with the activity sensor based upon programmed settings
and the respective sensor output.
Upper Rate (UR)--A value supplied by the clinician which limits the
maximum stimulation rate when the rate responsive mode for
activity, is in effect, such that the sensor-driven pacing rate
generated by pacemaker 100 does not become hemodynamically
excessive. With rate response, the allowed programmable values
range from 100 ppm to 175 ppm at 5 ppm intervals, provided UR must
also be at least 20 ppm greater than Lower Rate (LR).
The operation of the pacemaker 100 will now be described in greater
details in relation to FIGS. 3 and 4. FIG. 3 illustrates an
examplary activity attack and decay curve 300 indicative of the
behavior of the pacemaker 100, and FIG. 4 illustrates a simplified
flow chart of the software used to control the operation of the
pacemaker 100 according to the present invention.
The vertical axis in FIG. 3 represents the pacing rate in pulses
per minute (ppm) and the horizontal axis represents time in
seconds. Five threshold levels are illustrated as horizontal lines:
the upper rate 309; the achievement rate 310; the upper switch rate
311; the lower switch rate 312; and the lower rate 313.
As defined above, the upper rate 309 is a value supplied by the
physician which limits the maximum stimulation rate when the
activity reaches or exceeds a certain level. The pacemaker 100 is
not allowed to pace above the upper rate 309. The achievement rate
310 is a value that can be set by the physician and represents a
predetermined percentage of the difference between the upper rate
309 and the lower rate 313, as follows:
where "a" is a percentile value which preferably ranges between 50%
and 100%. The achievement rate 310 may vary from one patient to
another. However, for illustration purposes, an exemplary
achievement rate 310 is selected as 125 ppm.
The upper switch rate 311 is a value that can be selected by the
physician and represents a predetermined percentage of the
difference between the upper rate 309 and the lower rate 313, as
follows:
where "u" is a percentile value. The upper switch rate 311 varies
from one patient to another. However, the preferred range for the
upper switch rate 311 is between 20% and 50%. For illustration
purposes, the upper switch rate 311 is chosen as 90 ppm.
This upper switch rate 311 is an important factor in the present
invention in that it determines an upper switch point 326, which is
graphically represented as the intersection point between the
activity curve 300 and the upper switch rate 311. This upper switch
point 326 corresponds to the switch point 226 in FIG. 2.
The lower switch rate 312 is a value which is also selected by the
physician, and which represents a predetermined percentage of the
lower rate 313, as follows:
For illustration purposes, the lower switch rate threshold 312 is
chosen as 77 ppm.
The lower switch rate 312 is also an important factor in the
present invention in that it determines a lower switch point 328,
which is graphically represented as the intersection point between
the activity decay curve 300 and the lower switch rate threshold
312.
The lower rate 313 is a value supplied by the physician which
limits the minimum stimulation rate when the activity decreases to
or is below a certain level. The pacemaker 100 is not allowed to
pace below the lower rate 313. For illustration purposes, the lower
rate 313 is chosen as 70 ppm. While the above upper rate 309,
achievement rate 310, upper switch rate 311, lower switch rate 312
can be individually selected, their values can be set to default
values to simplify programming procedures.
The operation of the pacemaker 100 will now be described in
relation to the activity attack and decay curve 300 in FIG. 3.
While the operation of the pacemaker 100 will be described in
relation to a flow chart, it should be understood that the same or
a similar operation can be accomplished using conventional hardware
and integrated circuit technology. The initial point 324 indicates
that the patient is in a resting position and that the pacemaker
100 is pacing at the lower rate 313. When the patient is stressed
by exercise, the pacemaker 100 responds by increasing the pacing
rate, as illustrated by the attack curve 332, until it reaches a
maximum pacing rate or plateau 334, at which time, the pacing rate
stabilizes for the duration of the stress. The pacing rate 334 may
be limited by the upper rate 309.
If the patient maintains a heightened exercise level, and the
pacemaker 100 has paced above the achievement rate 310 for a
predetermined interval of time, such as 4 seconds or longer, then
the pacemaker 100 automatically triggers the inventive modified
decay feature, whereby the decay curve 335 is deflected at the
upper and lower switch points 326 and 328, as it reaches the upper
switch rate threshold 311 and the lower switch rate threshold 312
respectively. It should be understood that the waiting interval of
4 seconds could be varied for each patient, depending on age,
gender and activities which the patient engages into. Nonetheless,
the 4 second interval has been selected to substantially minimize
false triggering by artifacts.
Thus, upon decrease of the activity level, the pacemaker 100 is
allowed to pace at a decreasing rate, with a selectable decay time
constant such as 45 seconds. However, once the pacing rate reaches
the upper switch rate threshold 311, the decay time constant is
increased in order to slow the drop in the patient's pacing rate.
This modified decay feature simulates the heart's normal behavior
under the circumstances, and causes the pacemaker 100 to respond
optimally to the individual patient's cardiovascular needs.
The modified decay curve 330 generally corresponds to the latent
decay portion 230 in FIG. 2. If, prior to reaching the lower switch
rate threshold 312 the patient resumes a sudden heightened stress
or exercise level, then, as indicated by the deflection point 320,
the pacing rate increase correspondingly, as indicated by the
attack curve 322. It should be noted that, at this stage, since the
modified decay curve 330 has not reached the lower switch rate
threshold 312, the modified decay feature is still enabled, and has
not been turned off. The modified decay feature will be triggered
off when the pacing rate drops below the lower switch rate
threshold 312, along the curve 343, at which time, the modified
decay feature will not be enabled until the achievement criteria
have been met once again.
Therefore, as the pacing rate decay curve 337 reaches the upper
switch rate threshold 311, a corresponding switch point 339 causes
a change in the deceleration time constant. In this manner, the
pacing rate is allowed to decay along the decay curve 337 at the
time constant of 45 seconds, and upon reaching the upper switch
point 339, the pacing rate follows the modified decay curve
340.
The decay time constant of the modified decay curves 330 and 340
are substantially similar, and can be selected from a range of 90
to 180 seconds, with a preferred setting of 180 seconds. It should
however be understood to one skilled in the art after reviewing the
present disclosure that the decay curves 330 and 340 can have
different time constants, depending on the desired behavior of the
pacemaker 100.
Thus, in this particular example, since the attack curve has
reached and exceeded the achievement rate threshold 310, it might
be desirable to set the time constant of the decay curve 340 at a
value intermediate between the decay time constant of the curve
335, i.e. 45 seconds, and the modified time constant of the decay
curve 330, i.e. 180 second. By analogy, the decay time constant of
the curve 337 could also be selected to differ from the
conventional decay time constant of the curve 335, i.e. 45
seconds.
As the curve 340 reaches the lower switch rate threshold 312, its
decay time constant changes to a faster time constant 343, similar
to the conventional time constant of 45 seconds. A different time
constant can be selected.
Furthermore, in the preferred embodiment, upon reaching the lower
switch point 328, the modified decay achievement criterion is met,
such that, as long as the patient's exercise levels do not cause
the pacing rate to reach or exceed the achievement rate 310 for a
predetermined length of time, then the pacing rate is allowed to
decay at a nominal 45 seconds time constant.
This feature is illustrated by the attack curve 344 which falls
short of reaching the achievement rate threshold 310, and the decay
curve 347 is followed, even though the pacing rate decays below the
upper switch rate 311 and the lower switch rate 312. Hence, as
illustrated, the curve 347 is allowed to decay with a single non
interrupted time constant, since it is presumed that under such
circumstances the patient does not require additional time to
recoup from the increased sudden stress. Therefore, no deflection
is effected at the intersection points 349 and 351.
It should also be understood that the pacemaker 100 can be
programmed so that the intersection points 349 and 341 can become
switch points similar to the upper and lower switch points 339 and
328. In the alternative, the pacemaker 100 can be programmed to
cause the curve 347 to decay at a time constant different from that
of the decay curve 335. Such variations are anticipated by the
present description, and, for brevity purposes, they will not be
characterized in greater details.
Furthermore, it should also be understood to one skilled in the art
that one or more additional upper and lower switch levels can be
added between the achievement rate threshold 310 and the upper
switch rate threshold 311, as well as between the upper switch rate
311 and the lower rate 313, in order to generate a more gradual
deflection of the overall decay curve.
Turning now to FIG. 4, the operation of the pacemaker 100 will now
be described in greater detail in connection with the flow chart
400. The software program and/or hardware starts at 410, and then
determines, at 412, the new target rate, according to the following
equation: ##EQU2##
In the above equation, TR is the target rate calculated in response
to the activity sensor and C and D are programmable variables that
generate the shape of the rate response curves per FIG. 6.
The values of C and D, are generated by the pacemaker 100 or by an
external programmer (not shown) as a function of the selected upper
rate, selected lower rate and rate response setting and are
programmed into the memory or storage register of the pacemaker 100
using conventional programming techniques. The pacemaker 100
includes an arithmetic logic unit capable of making the necessary
calculations and controlling the rate of the pacemaker 100 based
upon the calculated target rate TR.
Each time the physician alters the selected upper rate, lower rate,
or rate response settings, the programmer generates a new set of
C-term, D-term, switch rates and achievement rate values, and loads
them into the memory or program registers of the pacemaker 100, so
that the arithmetic logic unit may calculate the target rate
thereafter based upon the updated values. Regardless of which of
the selected parameters has changed, the resulting function
relating the pacing rate to the sensor output will take the same
basic form, extending from the lower rate at a minimal sensor
output to the upper rate at an achievable sensor output, with a
sensor output required to achieve the upper rate decreasing as the
rate response setting is increased.
A more detailed description of the above calculations can be found
in the co-pending application, Ser. No. 455,717, entitled "METHOD
AND APPARATUS FOR IMPLEMENTING ACTIVITY SENSING IN A PULSE
GENERATOR", filed on Dec. 22, 1989, which is incorporated herein by
reference. Thus, as indicated at block 412, the pacemaker 100
periodically calculates the activity target rate TR, at a
two-second interval, along the curve 300 of FIG. 3.
Next, the software determines at 414, whether the modified decay
feature has been activated or programmed via a programmer such as
Medtronic Model 9760. If the modified decay feature has not been
activated, then, the software sets, at 415, the decay rate to be
equal to the programmed decay rate, i.e., in this example, 45
seconds time constant.
The pacemaker then calculates the next activty pacing rate at 416,
and saves the activity related data, at 417, for use in calculating
the new activity target rate at 412. The above routine is repeated
until the modified decay feature is activated.
If the modified decay feature has been enabled at 414, then, as
indicated by block 420, the software determines whether the
achievement criterion has been met, i.e. whether the pacing rate is
greater than or equal to the achievement rate 310, for a period of
4 seconds or longer. If the achievement criterion has not been met,
then the software sets, at 415, the decay rate to be equal to the
programmed decay rate, calculates the activity pacing rate at 416,
saves the activity data at 417, and then calculates the new
activity target rate at 412.
If on the other hand, the achievement criterion has been met, then,
as illustrated by block 433, the software determines whether the
current pacing rate is greater than the upper switch rate threshold
311. If it is, then, once again, the software sets, at 415, the
decay rate to be equal to the programmed decay rate, calculates the
activity pacing rate at 416, saves the activity data at 417, and
calculates the new activity target rate at 412.
If the pacing rate is less or equal to the upper switch rate 311,
then the software determines, at 437, whether the pacing rate is
between the upper switch rate 311 and the lower switch rate 312. If
it is, then, as indicated by block 439, the software changes the
decay rate to the modified or slower decay rate, as illustrated by
the decay curves 330 and 340 in FIG. 3. The activity pacing rate is
calculated at 416, the activity data is then saved at 417, and a
new activity target rate is calculated at 412.
If the pacing rate is less than the lower switch rate 312 then as
indicated by block 415, the software changes the decay rate to the
programmed value.
Turning now to FIG. 5, there is illustrated a block circuit diagram
500 of the pacemaker 100 utilizing a microprocessor with on-board
and off-board RAM/ROM memory and an activity sensor for adjusting
the physiologic pacing rate as a function of the patient activity.
The functional description of the circuit 500 is provided in detail
in the co-pending application Ser. No. 549,568, entitled "METHOD
AND APPARATUS FOR ACCESSING A NON-VOLATILE MEMORY", filed on Jul.
6, 1990, and having the same assignee as the present application,
and is incorporated herein by refererence.
Turning now to FIG. 6, there is illustrated a graph 600 of ten rate
response curves used in the pacemaker 100. The programmed rate
response setting establishes the relationship of the pacing rate to
the detected physical activity. The rate response has ten settings.
The most responsive setting (10) permits the greatest incremental
change in response to detected activity. The least responsive
setting (1) allows the smallest incremental change in response to
be detected by activity. When a change in the level of physical
activity is detected, the pacing will change gradually up and down,
as the case may be along the selected setting, i.e., (5). The
change is affected by a combination of several parameters, such as
activity threshold, rate response, upper and lower rates, and is
intended to imitate the rate responsiveness of the normal
heart.
The graph 600 is described in details in the co-pending
application, Ser. No. 455,717, entitled "METHOD AND APPARATUS FOR
IMPLEMENTING ACTIVITY SENSING IN A PULSE GENERATOR", filed on Dec.
22, 1989 and having the same assignee as the present application,
and is incorporated herein by reference.
While particular embodiments of the present invention have been
disclosed, it is to be understood that various different
modifications are possible and are contemplated within the scope of
the specification, drawings, abstract and appended claims.
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